1. Field of the Invention
This invention relates to x-ray lithography and more particularly to exposure of lithographic masks with x-ray radiation.
More particularly, it relates to use of the x-ray lithography masks to expose semiconductor circuit patterns through said masks.
2. Description of Related Art
In photolithography, light beams are employed to write upon photosensitive materials. Where masks are used, the writing light beam exposes patterns on photosensitive polymeric materials known as photoresist through patterned masks. The patterned masks comprise transparent and opaque regions for transmission of light through the mask or blocking of light. After exposure to the light, the exposed photoresist can be developed to reveal the exposed patterns. In industry today, one of the most important applications of such processes of exposing patterns onto photoresist is the manufacture of integrated semiconductor circuits. Such semiconductor circuits are being made smaller and smaller as the technology progresses. It has been the convention in industry in the recent past to use light in the visible range to expose semiconductor circuit patterns. However, the light wavelength should be about one half of pattern feature sizes to achieve adequate definition of image line width control. Accordingly, shorter wavelengths of light are being employed and now one trend is towards the use of x-rays to expose the resist with patterns for such small integrated circuits. X-ray radiation is employed to expose the photoresist through an x-ray mask. The type of radiation contemplated for a preferred embodiment of this invention is synchrotron x-ray radiation. The application of this invention is not limited to use with synchrotron x-ray radiation, however.
Synchrotron x-ray radiation is produced as a broad, fanned out horizontal beam which is narrow in the vertical direction. X-ray lithography systems are designed to take advantage of this form of fanned out radiation by passing relatively large horizontal angles (e.g. 25 mrad) through a high vacuum BL (beamline) and transmission window, and scanning the beam fan in the vertical direction. Large angle BL optics, and BL windows and x-ray masks with large dimensions are required to cover large fields in this fashion.
Synchrotron based x-ray lithography has the potential for forming microelectronic products by exposing images onto large fields of photoresist or the like at high speeds with concurrent high resolution. Realization of this potential, in practice, requires the ability to produce high resolution masks covering large fields with high positional accuracy.
In the manufacture of state of the art electronic chips by the technology of the exposure of photoresist with x-rays there is a basic problem below the half micron dimension, in that there is a conflict between the overlay requirement on the one hand and use of large field exposures, which large field exposures are conventionally thought to be the way to achieve high throughput on the other hand. There is a need to compromise between those two conflicting objectives, with accuracy but without sacrificing the speed of manufacture.
For example, a typical large field, large chip application would involve covering a 10 sq. cm field of five, 10 by 20 mm chips using a 50 mm horizontal BL aperture (and window) and would require an x-ray mask 50 mm wide and 20 mm high. In order to fabricate a large area mask of this large size from a membrane with sufficient mechanical stability to avoid being warped or bent requires that the membrane be sufficiently thick to avoid such distortion. In addition, in order to align and expose a single large field with high overlay requires that the needed positional accuracy of the mask be maintained over the entire large field.
If such large area masks could be made with adequate accuracy, stability and transmission of x-rays, they would also need to be fabricated at practical yields and throughput with very low defect levels. These combined requirements on large field masks are formidable and such masks may not be available in practice for a long time, if ever.
Steppers project a series of images from a single mask onto the wafer from a fixed location, with the mask separated, i.e. moved away from the wafer and returned into proximity with the wafer at a different position for each exposure. With a stepper, repeated exposures are made from the mask onto the wafer to fill the wafer with exposed images. When exposure of images is completed, then the wafer can be indexed to another position relative to the mask and the mask and wafer can be reexposed until the exposure is complete for the areas which were not exposed the first time.
There is a requirement for positional accuracy over an entire large field, but it is not possible, to achieve such standards at this time. Compounding the problem is the fact that future lithographic ground rule improvements, i.e. requiring a greater degree of miniaturization, will exacerbate the problem. Thick membranes are undesirable in that they reduce the quantity of energy transmitted through the membrane as a function of the thickness of the membrane. In addition, as the speed with which exposure is achieved is important, a commercially feasible method of achieving accuracy of exposure must perform with adequate speed for efficient production of chips.